Sql

1 Introduction to SQL
What is SQL?
– When a user wants to get some information
from a database file, he can issue a query.
– A query is a user–request to retrieve data or
information with a certain condition.
– SQL is a query language that allows user to
specify the conditions. (instead of algorithms)

1 Introduction to SQL
Concept of SQL

– The user specifies a certain condition.
– The program will go through all the records
in the database file and select those records
that satisfy the condition.(searching).
– Statistical information of the data.
– The result of the query will then be stored in
form of a table.

2 Basic structure of an SQL
query
G e n e ra l S E L E C T , A L L / D IS T IN C T , *,
S tru c tu re A S, FRO M , W H ERE

C o m p a ris o n IN , B E T W E E N , L IK E "% _"

G ro u p in g G R O U P B Y , H A V IN G ,
C O U N T ( ), S U M ( ), A V G ( ), M A X ( ), M IN ( )

D is p la y O rd e r O RD ER BY , A SC / D ESC

L o g ic a l AND, OR, NOT
O p e ra to rs

O u tp u t IN T O T A B L E / C U R S O R
T O F IL E [A D D IT IV E ], T O P R IN T E R , T O S C R E E N

U n io n U N IO N

2 The Situation:
Student Particulars
field type width contents
id numeric 4 student id number
name character 10 name
dob date 8 date of birth
sex character 1 sex: M / F
class character 2 class
hcode character 1 house code: R, Y, B, G
dcode character 3 district code
remission logical 1 fee remission
mtest numeric 2 Math test score

I General Structure

SELECT ...... FROM ...... WHERE ......
SELECT [ALL / DISTINCT] expr1 [AS col1], expr2 [AS col2] ;
FROM tablename WHERE condition

I General Structure
– The query will select rows from the source tablename
and output the result in table form.
– Expressions expr1, expr2 can be :
• (1) a column, or
• (2) an expression of functions and fields.

– And col1, col2 are their corresponding column names
in the output table.

I General Structure
– DISTINCT will eliminate duplication in the output
while ALL will keep all duplicated rows.
– condition can be :
• (1) an inequality, or
• (2) a string comparison
• using logical operators AND, OR, NOT.

I General Structure
Before using SQL, open the student file:
USE student
eg. 1 List all the student records.
SELECT * FROM student

id name dob sex class mtest hcode dcode remission
Result 9801 Peter 06/04/86 M 1A 70 R SSP .F.
9802 Mary 01/10/86 F 1A 92 Y HHM .F.
9803 Johnny 03/16/86 M 1A 91 G SSP .T.
9804 Wendy 07/09/86 F 1B 84 B YMT .F.
9805 Tobe 10/17/86 M 1B 88 R YMT .F.
: : : : : : : : :

I General Structure
eg. 2 List the names and house code of 1A students.

SELECT name, hcode, class FROM student ;
WHERE class="1A"

Class Class
1A  1A
1A
class="1A"
 1A
1A  1A
1B  1B
1B  1B
: :

I General Structure
eg. 2 List the names and house code of 1A students.

Result name hcode class
Peter R 1A
Mary Y 1A
Johnny G 1A
Luke G 1A
Bobby B 1A
Aaron R 1A
: : :

I General Structure
eg. 3 List the residential district of the Red House
members.
SELECT DISTINCT dcode FROM student ;
WHERE hcode="R"
dcode
Result
HHM
KWC
MKK
SSP
TST
YMT

I
eg. 4
General Structure
List the names and ages (1 d.p.) of 1B girls.

1B Girls ?

I
eg. 4
General Structure

Condition for "1B Girls":
1) class = "1B"
2) sex = "F"
3) Both ( AND operator)

I
eg. 4
General Structure

What is "age"?

I
eg. 4
General Structure

Functions:
# days : DATE( ) – dob
# years :(DATE( ) – dob) / 365
1 d.p.: ROUND(__ , 1)

I
eg. 4
General Structure

SELECT name, ROUND((DATE( )-dob)/365,1) AS age ;
FROM student WHERE class="1B" AND sex="F"

Result name age
Wendy 12.1
Kitty 11.5
Janet 12.4
Sandy 12.3
Mimi 12.2

I General Structure
eg. 5 List the names, id of 1A students with no fee
remission.
SELECT name, id, class FROM student ;
WHERE class="1A" AND NOT remission
name id class
Result Peter 9801 1A
Mary 9802 1A
Luke 9810 1A
Bobby 9811 1A
Aaron 9812 1A
Ron 9813 1A
Gigi 9824 1A
: : :

II Comparison
expr IN ( value1, value2, value3)
expr BETWEEN value1 AND value2
expr LIKE "%_"

II Comparison
eg. 6 List the students who were born on Wednesday
or Saturdays.
SELECT name, class, CDOW(dob) AS bdate ;
FROM student ;
WHERE DOW(dob) IN (4,7)
name class bdate
Result Peter 1A Wednesday
Wendy 1B Wednesday
Kevin 1C Saturday
Luke 1A Wednesday
Aaron 1A Saturday
: : :

II Comparison
eg. 7 List the students who were not born in January,
March, June, September.
SELECT name, class, dob FROM student ;
WHERE MONTH(dob) NOT IN (1,3,6,9)
name class dob
Result Wendy 1B 07/09/86
Tobe 1B 10/17/86
Eric 1C 05/05/87
Patty 1C 08/13/87
Kevin 1C 11/21/87
Bobby 1A 02/16/86
Aaron 1A 08/02/86
: : :

II Comparison

eg. 8 List the 1A students whose Math test score is
between 80 and 90 (incl.)
SELECT name, mtest FROM student ;
WHERE class="1A" AND ;
mtest BETWEEN 80 AND 90

Result name mtest
Luke 86
Aaron 83
Gigi 84

II Comparison
eg. 9 List the students whose names start with "T".

SELECT name, class FROM student ;
WHERE name LIKE "T%"

Result
name class
Tobe 1B
Teddy 1B
Tim 2A

II Comparison

eg. 10 List the Red house members whose names contain
"a" as the 2nd letter.

SELECT name, class, hcode FROM student ;
WHERE name LIKE "_a%" AND hcode="R"

Result name class hcode
Aaron 1A R
Janet 1B R
Paula 2A R

III Grouping
SELECT ...... FROM ...... WHERE condition ;
GROUP BY groupexpr [HAVING requirement]

Group functions:
COUNT( ), SUM( ), AVG( ), MAX( ), MIN( )

– groupexpr specifies the related rows to be grouped
as one entry. Usually it is a column.
– WHERE condition specifies the condition of
individual rows before the rows are group.
HAVING requirement specifies the condition
involving the whole group.

III Grouping
eg. 11 List the number of students of each class.

Group By Class
class
1A

1A 1A
1A
COUNT( )

1B
1B
1B COUNT( )
1B 1B
1B
1B
1C

1C 1C
1C
COUNT( )

Student

III Grouping
eg. 11 List the number of students of each class.
SELECT class, COUNT(*) FROM student ;
GROUP BY class
class cnt
Result 1A 10
1B 9
1C 9
2A 8
2B 8
2C 6

III Grouping
eg. 12 List the average Math test score of each class.

Group By Class
class
1A

1A 1A AVG( )
1A
1B
1B

1B 1B AVG( )
1B
1B
1B
1C

1C 1C AVG( )
1C
Student

III Grouping
eg. 12 List the average Math test score of each class.

SELECT class, AVG(mtest) FROM student ;
GROUP BY class

class avg_mtest
Result 1A 85.90
1B 70.33
1C 37.89
2A 89.38
2B 53.13
2C 32.67

III Grouping
eg. 13 List the number of girls of each district.

SELECT dcode, COUNT(*) FROM student ;
WHERE sex="F" GROUP BY dcode

Result dcode cnt
HHM 6
KWC 1
MKK 1
SSP 5
TST 4
YMT 8

III Grouping
eg. 14 List the max. and min. test score of Form 1
students of each district.
SELECT MAX(mtest), MIN(mtest), dcode ;
FROM student ;
WHERE class LIKE "1_" GROUP BY dcode
max_mtest min_mtest dcode
Result 92 36 HHM
91 19 MKK
91 31 SSP
92 36 TST
75 75 TSW
88 38 YMT

III Grouping
eg. 15 List the average Math test score of the boys in
each class. The list should not contain class with
less than 3 boys.
SELECT AVG(mtest), class FROM student ;
WHERE sex="M" GROUP BY class ;
HAVING COUNT(*) >= 3
Result avg_mtest class
86.00 1A
77.75 1B
35.60 1C
86.50 2A
56.50 2B

IV Display Order
GROUP BY ..... ;
ORDER BY colname ASC / DESC

IV Display Order
eg. 16 List the boys of class 1A, order by their names.

SELECT name, id FROM student ;
WHERE sex="M" AND class="1A" ORDER BY name

name id Result name id
Peter 9801 Aaron 9812
Johnny 9803 ORDER BY Bobby 9811
Luke 9810 Johnny 9803
dcode
Bobby 9811 Luke 9810
Aaron 9812 Peter 9801
Ron 9813 Ron 9813

IV Display Order
eg. 17 List the 2A students by their residential district.
SELECT name, id, class, dcode FROM student ;
WHERE class="2A" ORDER BY dcode
name id class dcode
Result Jimmy 9712 2A HHM
Tim 9713 2A HHM
Samual 9714 2A SHT
Rosa 9703 2A SSP
Helen 9702 2A TST
Joseph 9715 2A TSW
Paula 9701 2A YMT
Susan 9704 2A YMT

IV Display Order
eg. 18 List the number of students of each district
(in desc. order).
SELECT COUNT(*) AS cnt, dcode FROM student ;
GROUP BY dcode ORDER BY cnt DESC
cnt docode
Result 11 YMT
10 HHM
10 SSP
9 MKK
5 TST
2 TSW
1 KWC
1 MMK
1 SHT

IV Display Order
eg. 19 List the boys of each house order by the
classes. (2-level ordering)

SELECT name, class, hcode FROM student ;
WHERE sex="M" ORDER BY hcode, class

IV Display Order
Result
name hcode class
Blue
Bobby B 1A
House Teddy B 1B Order
Joseph B 2A by
Zion B 2B class
Order
Leslie B 2C
by Johnny G 1A
hcode Luke G 1A
Kevin G 1C
Green
House George G 1C
: : :
:
:

V Output

IN T O T A B L E tablen am e the output table is saved as a
database file in the disk.

IN T O C U R S O R tem p the output is stored in the
w orking m em ory tem porarily.

T O F IL E filen am e [A D D IT IV E ] output to a text file.
(additive = append)

T O P R IN T E R send to printer.

TO SC R EEN display on screen .

V
eg. 20
Output
List the students in desc. order of their names and
save the result as a database file name.dbf.
SELECT * FROM student ;
ORDER BY name DESC INTO TABLE name.dbf
Result id name dob sex class mtest hcode dcode remission
9707 Zion 07/29/85 M 2B 51 B MKK .F.
9709 Yvonne 08/24/85 F 2C 10 R TST .F.
9804 Wendy 07/09/86 F 1B 84 B YMT .F.
9819 Vincent 03/15/85 M 1C 29 Y MKK .F.
9805 Tobe 10/17/86 M 1B 88 R YMT .F.
9713 Tim 06/19/85 M 2A 91 R HHM .T.
9816 Teddy 01/30/86 M 1B 64 B SSP .F.
: : : : : : : : :

V
eg. 21
Output
Print the Red House members by their classes, sex
and name.
SELECT class, name, sex FROM student ;
WHERE hcode="R" ;
ORDER BY class, sex DESC, name TO PRINTER
class name sex
Result 1A Aaron M
1A Peter M
1A Ron M
1B Tobe M
1B Janet F
1B Kitty F
1B Mimi F
: : :

3 Union, Intersection and
Difference of Tables
The union of A and B (A B)

A B

A table containing all the rows from A and B.

The intersection of A and B (A B)

A B

A table containing only rows that appear in both A and B.

The difference of A and B (A–B)

A B

A table containing rows that appear in A but not in B.

3 The Situation:
Bridge Club & Chess Club
Consider the members of the Bridge Club and
the Chess Club. The two database files have
the same structure:

name character 10 name
sex character 1 sex: M / F
class character 2 class

B rid ge [A ] C hess [B ]
id nam e sex class id nam e sex class

1 9812 A aron M 1A 1 9802 M ary F 1A
2 9801 P eter M 1A 2 9801 P eter M 1A
3 9814 K enn y M 1B 3 9815 E dd y M 1B
4 9806 K itty F 1B 4 9814 K enn y M 1B
5 9818 E dm ond M 1C 5 9817 G eorge M 1C
: : : : : : : :

Before using SQL, open the two tables:
SELECT A
USE bridge
SELECT B
USE chess

SELECT ...... FROM ...... WHERE ...... ;
UNION ;

eg. 22 The two clubs want to hold a joint party.
Make a list of all students. (Union)
SELECT * FROM bridge ;
Result
UNION ;
SELECT * FROM chess ;
ORDER BY class, name INTO TABLE party

SELECT ...... FROM table1 ;
WHERE col IN ( SELECT col FROM table2 )

eg. 23 Print a list of students who are members of both
clubs. (Intersection)

Result SELECT * FROM bridge ;
WHERE id IN ( SELECT id FROM chess ) ;
TO PRINTER

SELECT ...... FROM table1 ;
WHERE col NOT IN ( SELECT col FROM table2 )

eg. 24 Make a list of students who are members of the
Bridge Club but not Chess Club. (Difference)

Result SELECT * FROM bridge ;
WHERE id NOT IN ( SELECT id FROM chess ) ;
INTO TABLE diff

4 Multiple Tables:
• SQL provides a convenient operation to
retrieve information from multiple tables.

• This operation is called join.

• The join operation will combine the tables into
one large table with all possible combinations
(Math: Cartesian Product), and then it will filter
the rows of this combined table to yield useful
information.

4 Multiple Tables:

field1 field2
A 1
field1 field2
A 2
A 1
A 3
B 2
B 1
3
B 2
B 3

4 The Situation:
Music Lesson
Each student should learn a musical instrument.
Two database files: student.dbf & music.dbf
The common field: student id

type character 10 type of the music instrument

SELECT A
USE student
SELECT B
USE music

4 Natural Join
A Natural Join is a join operation that joins two
tables by their common column. This operation
is similar to the setting relation of two tables.

SELECT a.comcol, a.col1, b.col2, expr1, expr2 ;
FROM table1 a, table2 b ;
WHERE a.comcol = b.comcol

4
eg. 25
Natural Join
Make a list of students and the instruments they
learn. (Natural Join)
id name class id type

Same id 9801
9801

Join
Student Music

id name class type

9801
Product

4
eg. 25
Natural Join
Make a list of students and the instruments they
learn. (Natural Join)
SELECT s.class, s.name, s.id, m.type ;
FROM student s, music m ;
WHERE s.id=m.id ORDER BY class, name
class name id type
Result 1A Aaron 9812 Piano
1A Bobby 9811 Flute
1A Gigi 9824 Recorder
1A Jill 9820 Piano
1A Johnny 9803 Violin
1A Luke 9810 Piano
1A Mary 9802 Flute
: : : :

4 Natural Join
eg. 26 Find the number of students learning piano in
each class.

Three Parts :
(1) Natural Join.
(2) Condition: m.type="Piano"
(3) GROUP BY class

4 Natural Join
eg. 26

Student Join Condition Group By
m.type= "Piano" class
Product

Music

4 Natural Join
eg. 26 Find the number of students learning piano in
each class.
SELECT s.class, COUNT(*) ;
WHERE s.id=m.id AND m.type="Piano" ;
GROUP BY class ORDER BY class
class cnt
Result 1A 4
1B 2
1C 1

4 Outer Join
An Outer Join is a join operation that includes
rows that have a match, plus rows that do not
have a match in the other table.

4 Outer Join
eg. 27 List the students who have not yet chosen an
instrument. (No match)

id name class id type

9801
No match

Student Music

4 Outer Join
eg. 27 List the students who have not yet chosen an
instrument. (No match)
SELECT class, name, id FROM student ;
WHERE id NOT IN ( SELECT id FROM music ) ;
ORDER BY class, name
class name id
Result 1A Mandy 9821
1B Kenny 9814
1B Tobe 9805
1C Edmond 9818
1C George 9817
: : :

4 Outer Join
eg. 28 Make a checking list of students and the
instruments they learn. The list should also
contain the students without an instrument.
(Outer Join)

4 Outer Join
eg. 28

Natural Join

Outer Join

No Match

4 Outer Join
eg. 28 SELECT s.class, s.name, s.id, m.type ;
WHERE s.id=m.id ;
UNION ;
SELECT class, name, id, "" ;
FROM student ;
WHERE id NOT IN ( SELECT id FROM music ) ;
ORDER BY 1, 2

4
class
Outer Join
name id type
class name id type
1A Aaron 9812 Piano
1A Bobby 9811 Flute 1A Aaron 9812 Piano
1A Gigi 9824 Recorder 1A Bobby 9811 Flute
1A Jill 9820 Piano 1A Gigi 9824 Recorder
1A Johnny 9803 Violin 1A Jill 9820 Piano
1A Luke 9810 Piano 1A Johnny 9803 Violin
1A Mary 9802 Flute 1A Luke 9810 Piano
: : : : 1A Mandy 9821
Natural Join 1A Mary 9802 Flute
1A Peter 9801 Piano
class name id
1A Mandy 9821
1A Ron 9813 Guitar empty
1B Eddy 9815 Piano
1B Kenny 9814
1B Tobe 9805 1B Janet 9822 Guitar
1C Edmond 9818 1B Kenny 9814
1C George 9817 1B Kitty 9806 Recorder
: : : : : : :
No Match Outer Join

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